Implantable medical devices often try to measure as many physiological parameters as possible with components already available in the implantable device or with minimal changes to the existing hardware. This is especially true when a device, such as an implantable cardioverter-defibrillator (ICD), contains an impressive array of electronic hardware and programmable software components. These components can be leveraged to shed light on many patient medical conditions without extra design cost. Thus, these components are sometimes used for measuring physiological variables not directly related to the primary cardiac functionality of an ICD, such as measuring changes in thoracic impedance in order to track respiration.
Because ability to measure some of these physiological parameters has been added to implantable devices as an afterthought, it can happen that such measurements are not performed in the best manner, since an implantable device retrofitted to perform ancillary measurements is intended primarily for a different purpose.
Some conventional devices try to analogize impedance results in the body from simple resistance measurements between two points in the body. These results are suspect because there are few electrical pathways in the body with impedances that can be reliably described by simple resistance measurements. These resistance measurements sometimes try to measure hemodynamic variables, cardiac parameters, presence of edema, tissue changes, etc. Measurement of these parameters places a heavy burden on crude resistance measurements, especially if the electrical pulses used to perform the resistance measurements change the parameter being measured or if the conventional implanted device does not properly filter out extraneous influences that interfere with such measurements. Thus, some conventional techniques used by implanted devices for deriving impedance values in the body are half-hearted or unsophisticated attempts at making what should be a more thorough measurement, and thus result in inaccuracy and low reliability.
The assumption that a resistance measurement is truly describing a bodily impedance can result in some false negatives. For example, a pathological condition being tracked may really be present, but the conventional implanted device does not detect it due to a lack of sensitivity of the measurements or because the conventional technique examines only a limited pathway of tissue.
Conventional reliance on a resistance component of impedance may also lead an implanted device to false positives, in which measurements indicate presence of the pathological condition, but the conventional technique is actually measuring something else entirely. For example, a conventional device may send out a sensing signal that causes a change in an ionic balance, which the conventional device then erroneously interprets as a change in the physiological parameter being measured.
Trying to sample a signal at points in time that are contrived to coincide synchronously with applied pulses is a very unreliable conventional pitfall. This conventional approach is nearly impossible to successfully implement, because the effects of phase delay, cardiac cycle, respiratory cycle, etc., would have to be known beforehand to successfully synchronize “snapshot” sampling measurements with the timing of applied pulses. These influencing effects that need to be known beforehand are actually components of the parameter being measured, thus the synchronization is typically faulty and the obtained measurements are inaccurate.